This application relates generally to the field of coupling devices for electrical circuits and in particular to the field of directional couplers.
Directional couplers which include parallel microstrip conductors mounted on a dielectric, commonly referred to as microstrip couplers, are widely used in various types of circuits, including high frequency RF (radio frequency) and microwave circuits. Microstrip couplers are often used in connection with signal sampling (power monitoring), signal splitting and combining, signal injection and other applications.
If a directional coupler is not properly terminated, reflected waves travel back from the load to the input. These reflected waves cause degradation in the performance of the system. In a type of conventional microstrip coupler called a Lange coupler, wire or ribbon conductors are typically used to form xe2x80x9ccontrolled capacitance bridges.xe2x80x9d Controlled capacitance bridges are often used to connect alternating split microstrip conductors and these bridges typically reduce parasitic inductance. However, there is typically a parasitic capacitance associated with an controlled capacitance bridge that is not easily controlled. Such parasitic capacitance affects the circuit performance adversely. Since this capacitance affects coupler performance, it is desirable to control the amount of capacitance present and account for the amount of capacitance present while designing the coupler. The Lange coupler is described in U.S. Pat. No. 3,516,024 (xe2x80x9cthe Lange patentxe2x80x9d), which is hereby incorporated by reference.
The characteristic impedance of a microstrip coupler is a function of the product of the impedances of the even and odd modes of TEM transmission. The degree of coupling is a function of the ratio of the even and odd mode impedances. Odd and even mode phase velocities in the microstrip conductors are not equal and this difference in velocity leads to poor directivity. The directivity generally becomes worse as the coupling is decreased. As will be appreciated by those skilled in the art, a compensating capacitor is typically placed between one or more coupled microstrip conductors and an input microstrip conductor to improve directivity.
Accordingly, port impedance, coupling, and directivity are important characteristics that need to be considered in the design of a directional coupler in order to achieve proper termination. However, in a conventional broadside-coupled directional coupler, the coupling and matching port impedance cannot be independently adjusted. As a result, circuit designers must often abandon the directional coupler approach and use alternative circuit designs, or use an additional matching circuit to complete a circuit design. Thus, it would be desirable to provide a coupler that utilizes a controlled parasitic capacitance bridge in providing a coupler having improved directivity.
According to one aspect of the presently-claimed invention, a microstrip coupler includes: a first microstrip conductor configured to carry an input signal; a second microstrip conductor disposed along a first side of the first microstrip conductor and configured to couple at least a portion of the input signal; a third microstrip conductor disposed along a second side of the first microstrip conductor and configured to couple at least a portion of the input signal; and a first controlled capacitance bridge connecting the second microstrip conductor and the third microstrip conductor. The controlled capacitance bridge includes a conducting layer and a dielectric layer situated between the conducting layer and the first microstrip conductor.
According to another aspect of the present invention, an controlled capacitance bridge is provided for connecting a first microstrip conductor and a second microstrip conductor of a microstrip coupler. The first microstrip conductor is disposed along a first side of a third microstrip conductor configured to carry an input signal and the second microstrip conductor is disposed along a second side of the third microstrip conductor. The controlled capacitance bridge includes a conducting layer and a dielectric layer situated between the conducting layer and the third microstrip coupler.
According to another aspect of the present invention, a microstrip coupler includes: an input microstrip conductor configured to carry an input signal; a central microstrip conductor proximate the input microstrip conductor and separated from the input microstrip conductor by a first gap; an output microstrip conductor proximate the central microstrip conductor and separated from the central microstrip conductor by a second gap; a coupling microstrip conductor for coupling at least a portion of the input signal A first controlled capacitance bridge connects the input microstrip conductor and the central microstrip conductor. The first controlled capacitance bridge includes a first conducting layer and a first dielectric situated between the first conducting layer and the first gap. A second controlled capacitance bridge connects the central microstrip conductor and the output microstrip conductor. The second controlled capacitance bridge includes a second conducting layer and a second dielectric situated between the second conducting layer and the second gap.